Man-made structures such as boat hulls, buoys, drilling platforms, oil production rigs, and pipes which are immersed in water are prone to fouling by aquatic organisms such as green and brown algae, barnacles, mussels, and the like. Such structures are commonly of metal, but may also comprise other structural materials such as concrete. This fouling is a nuisance on boat hulls, because it increases frictional resistance during movement through the water, the consequence being reduced speeds and increased fuel costs. It is a nuisance on static structures such as the legs of drilling platforms and oil production rigs, firstly because the resistance of thick layers of fouling to waves and currents can cause unpredictable and potentially dangerous stresses in the structure, and, secondly, because fouling makes it difficult to inspect the structure for defects such as stress cracking and corrosion. It is a nuisance in pipes such as cooling water intakes and outlets, because the effective cross-sectional area is reduced by fouling, with the consequence that flow rates are reduced.
The commercially most successful methods of inhibiting fouling have involved the use of anti-fouling coatings containing substances toxic to aquatic life, for example tributyltin chloride or cuprous oxide. Such coatings, however, are being regarded with increasing disfavour because of the damaging effects such toxins may have if released into the aquatic environment. There is accordingly a need for non-fouling coatings which do not contain markedly toxic materials.
It has been known for many years, for example as disclosed in GB 1,307,001 and U.S. Pat. No. 3,702,778 that silicone rubber coatings resist fouling by aquatic organisms. It is believed that such coatings present a surface to which the organisms cannot easily adhere, and they can accordingly be called non-fouling rather than anti-fouling coatings. Silicone rubbers and silicone compounds generally have very low toxic properties. Silicone rubber coatings have, however, gained little commercial acceptance. It is difficult to make them adhere well to the substrate surface that is to be protected, and mechanically they are rather weak and liable to damage.
It is known to use fluorinated polymers for fouling control in anti-fouling or non-fouling coating compositions.
In JP 04-045170 a fluorinated silicone resin is disclosed which is obtained by grafting a fluorine-containing acrylate to a silicone resin having olefinically unsaturated bonds in its terminal groups.
In JP 61-043668 a coating composition having antifouling properties is disclosed which is prepared by compounding an alkyd resin with a polymer prepared by reacting a fluorine-containing monomer with an acrylate polymer.
In JP 06-322294 a corrosion protecting antifouling coating is disclosed comprising a film forming resin and an organopolysiloxane having oxyalkylene groups and perfluoroalkyl groups.
Fluorinated polymers are also known for other uses.
In JP 06-239876 a fluorinated polymer having excellent wetting properties is disclosed that is used in an adhesive. In U.S. Pat. No. 4,900,474 a perfluoroether group-containing organopolysiloxane is disclosed that is used as a silicone antifoamer.
None of the fluorinated polymers that are known in the art have found wide application in antifouling coating compositions, since their anti-fouling/foul release properties are not sufficient and/or their mechanical properties do not make these compositions suited for use on various kind of structures that are immersed in water. In particular, the mechanical properties should be such that if applied as a coating composition for a boat hull, said coating composition should have sufficient strength and abrasion resistance to have a service life of several years.